We have used a gene encoding a stable, biologically active suppressor tRNA as a reporter of transcription. We used this RNA reporter to show that the bacteriophage HK022 Nun protein blocks gene expression by terminating transcription close to the phage ~ nut sites, and does not act by destabilizing transcripts or by preventing translation. This reporter system should be useful for other investigations in which it is necessary to distinguish between transcript termination and transcript stability. We have continued our studies on the mechanism of transcription antitermination in bacteriophage HK022. Measurements of terminator read- through in vivo and in vitro suggest that antitermination requires a Zinc- binding domain in the ~' subunit of E. coli RNA polymerase, and a specific site that lies upstream of terminator signals in the two early transcription units of the phage. The protein domain has been identified by a cluster of mutations that prevent antitermination, but otherwise leave polymerase activity nearly unaltered. We have found that these mutations also alter expression of several bacterial transcription units, and we are isolating and characterizing the genes involved. The integrase proteins of phages ~ and HK022 are closely related site-specific recombin- ases that recognize different nucleotide sequences in their substrates, the attachment sites of the two phages. To locate the determinants of specificity within the sites and proteins, we have developed a new excision assay for rapidly testing the activities of mutants. We find that a chimeric protein that contains only 12 amino acid residues of HK022 origin has nearly complete HK022 specificity. These residues lie between positions 279 and 330, overlapping the active site of the protein. Recip- rocal chimeras do not have the reciprocal phenotype, but instead recognize both types of sites. We also found that pairs of sites with 4 identical core regions are less active than wild type sites. We propose a model in which cores are recognized by an Integrase oligomer, and that interactions between the protomers indirectly affect DNA site recognition.